The present invention relates to an apparatus for wrapping groups of products with plastic film, especially heat-shrink film.
The apparatus applies to the field of product packaging machines, with particular reference to the treatment of products of cylindrical or parallelepiped shape, such as bottles, cans and the like.
At present, automatic machines of this type present a line for processing and feeding bottles (this specific term being used for convenience of description, without thereby excluding products of a different shape) to a series of stations for positioning and collating the products, and for wrapping the resulting product groups with plastic film.
The positioning and collating stations are designed to form groups with a preset number of products, which may vary according to the size of the product and of the package to be made (in the case of bottles, for example, each group may consist of two rows of three bottles each), and to convey the products to the wrapping stations, at the end of which each group of products formed leaves the station and is fed to an end station where the packaging process is completed, usually with heat-shrink film.
The wrapping station includes a roll feed system constituting a station for continuously feeding heat-shrink plastic film (used to wrap the products) and located at a level below the conveyor along which the products are moving.
The roll feeds a unit for defining and cutting the length of film necessary to wrap the group of products in a direction corresponding to the direction of product feed as far as the product conveyor.
According to known methods, a wrapping “bridge” on the conveyor guides the film (suspended on crosspieces driven by parallel chains on both sides of the crosspieces themselves) over the product groups, passing through a slot, in such manner as to wrap the product group as it moves along the conveyor.
In practice, the group of products is wrapped by the film being unwound in the same direction as the products are being fed, the ends of the film partially overlapping after the wrapping operation and leaving an opening in the wrapping at the vertical lateral faces of the group itself. The tightening of the film over the product is accomplished by the typical heat-shrink properties of the film, as is very well known in the trade.
Up to now, based on this machine structure, attempts have been made to improve productivity and to reduce down time, for example, for size changeover, requiring various operations on the drive units (such as those of the unit which feeds and cuts the film, so as to change the size of the film lengths), and mechanical down time, for example, due to adjustments with manual means in the height of the structure which drives the film lifting and guide crosspieces.
For this reason, the Applicant designed and built a feed and cutting unit (see patent EP 839.723) located immediately below the slot for film feed to the wrapping surface and consisting of a pair of counter-rotating rollers between which the film to be fed is inserted, at a tangential point of contact. The first roller has a radially projecting blade, whilst the second roller has a radial groove which accommodates the blade during a cutting rotation, and a flat facet on its surface, forming a channel for the free transit of the film.
This structure significantly improves film feed and cutting, making it almost continuous, and speeds up changeover procedures in terms of the length and formation of the length of film to be used on the groups of products.
Following these improvements to the film feed and forming unit, the Applicant has also designed and built an improved unit for wrapping the groups of products (see Italian patent application BO2001A000284). This improved unit consists of at least one crosspiece for supporting and guiding the film, supported at one end by a single, motor-powered bar, positioned close to one side of the conveyor surface, and adjusting means acting at least upon the single bar and designed to allow a rotation of the crosspiece between a first, active trajectory, extending over the conveyor surface, and a second, return trajectory, extending under the conveyor surface, following an arc-shaped path whose distance from the conveyor surface varies and may be preset according to the dimensions of the group of products to be wrapped.
This constructional architecture has made it possible to further increase wrapping speed and reduce changeover down time, connecting the drive and control system of the wrapping unit to the film feed and cutting unit, the latter consisting of pairs of feed rollers mounted on both sides of a film cutting unit.
After numerous tests, however, it has now been found that the increased wrapping speed and faster changeover time have upset the normal functioning of the traditional station for unwinding the film from the roll.
Indeed, in current machines, the pre-unwinding station consists of the aforementioned roll of film and of a roller (usually known as “dandy roller) that moves with rocking motion, located downstream of the roll in the unwinding direction of the film and acting as a feed buffer keeping the cutting unit supplied with film.
Since the operation of the cutting unit depends on the system described above, it is subjected to irregular requests for film feed, due above all to the faster rate at which the groups of products are wrapped and which is not compensated by the pre-unwinding unit.
This irregularity may cause the film to be elastically deformed or move off its feed path as it travels from the dandy roller to the cutting station just under the working surface.
This may lead to unwanted waviness or folds in the film being fed to the cutting unit, which may in turn result in the film being cut erratically and to incorrect lengths.
Moreover, the area close to the cutting unit is, used for centering the printed spots on the film so that the latter is correctly positioned on the group of products: it is evident that deformation or waviness in the film prevents the printed spots from being correctly positioned when the length of film is cut.
The present invention therefore has for an aim to overcome the above mentioned drawbacks through an apparatus for wrapping groups of products with plastic film structured in such a way as to provide high productivity and to significantly reduce changeover down time while at the same time offering a high degree of film unwinding precision suited to the productivity of the apparatus, but without altering the basic structure of the apparatus itself.